1. Field of the Invention
The present invention pertains to a method of limiting the current implemented in a speed controller for three-phase asynchronous electric motor operating according to a control law of U/F type. The present invention also relates to a system for limiting the current that can implement such a method.
A conventional speed controller for asynchronous motor comprises in particular a voltage inverter, for example of PWM (Pulse Width Modulation) type, associated with an uncontrolled rectifier and with a capacitive filter.
2. Description of the Prior Art
It is known that a speed controller associated with an asynchronous motor has a tendency to provide more current to the motor when the motor is subject to an overload. However, in such a situation, the power components of the inverter of the controller may be overly stressed and their lifetime may be affected thereby. It is therefore necessary to limit the current delivered to the motor so as to avoid damaging these power components. Accordingly, in a conventional speed controller operating according to a voltage control law, for example of vector type, use is made of a speed regulator associated with a speed loop and a current regulator associated with a current loop.
Now, in a speed controller operating according to a voltage control law of pure U/F type, no current loop or speed loop is tolerated. Such a speed controller is admittedly less effectual but proves to be much more robust and may in particular be used in products such as transformers or fans.
Patent Application EP 1 229 637 (corresponding to patent U.S. Pat. No. 6,680,599) discloses a system for limiting the current output by a speed controller for asynchronous motor. Such a system is used to limit the current in case of overload in a controller operating according to any voltage control law whatsoever. With reference to FIG. 2 appended, such a system of the prior art operates in the following manner:
A microcontroller circuit Mc01 periodically receives the current signals provided by sensors in the phases of the motor M. These current signals are thereafter processed by a current limitation circuit implementing a current limitation function LIC01. The current limitation function LIC01 determines firstly the modulus of the vector of the phase currents ModI of the motor M, designated hereinbelow modulus of the current vector. It then compares this modulus ModI with a limit current value Ilim stored in the controller V01 and calculates the limitation error. On the basis of this limitation error, the limitation function LIC01 calculates, with the aid of an integral action regulator PI, a control variable. The limitation function LICO1 then multiplies this control variable with the phase currents measured in such a way as to obtain a correction voltage dV for each phase. This correction voltage dV is then deducted from the voltage Vqref calculated according to the control law LCO1 used for the operation of the motor M so as to obtain a corrected reference voltage Vqrefc. Thus, at the output of the speed controller V01, a reference voltage Vqrefc, corrected by virtue of the limitation function LIC01, is applied to the motor M via an inverter PWM.
As represented in FIG. 2, when the voltage control law LC01 used by the speed controller is of U/F type, that is to say with no current loop or speed loop, the voltage Vqref obtained according to the voltage control law of the motor is proportional to the frequency of the stator Wstat in accordance with a determined constant k such that Vqref=k*Wstat and the frequency of the stator Wstat is equal to the frequency reference Wref defined at the input of the speed controller. The constant k represents the magnetic flux created by the magnetizing current flowing through the stator. When a limitation function LIC01 such as that described hereinabove is implemented to limit the current in case of an overload in a speed controller operating according to a voltage control law of U/F type, it provides a correction voltage dV such that the corrected reference voltage Vqrefc calculated, equal to the voltage applied across the terminals of the stator, is equal to k*Wstat−dV. Thus this corrected voltage Vqrefc ensures accurate limitation of the current delivered to the motor M.
Nevertheless, in this case, the voltage equal to Vqrefc which is applied to the terminals of the stator decreases but the frequency of the stator Wstat remains identical. Thus the torque that can be provided by the motor, which is substantially proportional to the U/F ratio, that is to say to Vqrefc/Wstat, also decreases. The torque of the motor during the current limitation is thus less than the torque in the normal situation. DURING a current limitation performed with the aid of a limitation function LIC01 described in application EP 1 229 637, the performance of a motor operating according to a voltage control law of U/F type is diminished and the motor may even stall.